Initial Inspection and Diagnostic Overview
Four-Wheel Drive Systems
Special Tool(s)
Principles of Operation
The Automatic Four-Wheel Drive (A4WD) system is an electronic-shift 4WD system that allows the operator to choose between 2WD and three different 4WD modes. The operator can switch between A4WD and 4WD HIGH modes at any speed. To engage or disengage 4WD LOW range, the vehicle speed must be less than 3 Km/h (2 mph), the brake pedal must be pressed and the transmission must be in NEUTRAL.
Wheel slip is sensed using one axle shaft speed sensor on each axle. Based on throttle position and wheel slip, a Pulse Width Modulated (PWM) signal is transmitted to the transfer case clutch when a predetermined slip-threshold is exceeded.
The 4WD control module (PCM) will provide the IVD brake system with current clutch duty cycle and whether or not IVD may command the clutch duty cycle.
The brake subsystem sends the following information signals to the 4WD system:
^ Occurrences of brake events and failures.
^ Steering wheel angle status.
^ Average rear wheel speed.
^ Average front wheel speed.
^ Delta front wheel speeds.
A concern with any of the above listed brake system signals will affect operation of the 4WD system.
The throttle position output signal is provided to the 4WD control module from the Powertrain Control Module (PCM). This signal is used by the 4WD control module in controlling the 4WD clutch.
The shift motor sense plate, an integral part of the gear motor encoder assembly, informs the 4WD control module of the transfer case position.
The digital Transmission Range (TR) sensor is located on the LH side of the transmission. This sensor informs the 4WD control module when the transmission is in NEUTRAL.
The electric shift motor is mounted externally to the gear motor encoder assembly at the rear of the transfer case. It drives a rotary cam which moves the mode fork and range fork within the transfer case between the HIGH range (A4WD, 4WD HIGH) and 4WD LOW range positions.
The 4WD shift motor is controlled by the 4WD control module which shifts the transfer case shift motor between HIGH range (A4WD, 4WD HIGH) and 4WD LOW modes.
The solid-state clutch is duty-cycled by the 4WD control module to activate the 4WD clutch within the transfer case.
In the A4WD system, the 4WD control module varies the torque sent to the front driveline by controlling the transfer case clutch. At rest and under cruising conditions, the 4WD control module activates the transfer case clutch a minimum duty cycle (percentage of time the clutch is turned on), which allows for the slight difference between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. Under any of the following conditions, the 4WD control module will increase the duty cycle in order to prevent or control slip:
^ Slip is detected
^ Heavy acceleration (throttle position)
Feature inputs:
^ Brake ON/OFF switch
^ Mode Select Switch (MSS)
^ Wheel speed signal (transmitted from the ABS module)
^ Throttle position information (transmitted from the PCM)
^ Gear motor encoder contact plate position inputs A, B, C, D C, D
Feature outputs:
^ Solid state clutch (pulse width modulated signal)
^ 4WD LOW indicator
^ 4WD shift motor outputs
Shifts between A4WD and 4WD HIGH can be made at any speed. Listed below are the inputs and outputs needed by the 4WD control module to execute a change between any of these modes.
Feature inputs:
^ Wheel speed signal
^ Throttle position information
^ 4WD Mode Select Switch (MSS)
Feature outputs:
^ Solid state clutch (pulse width modulated signal)
^ 4WD HIGH switch indicator
When shifting into or out of LOW range, the 4WD control module requires that the vehicle speed is less than 3 Km/h (2 mph), the brake is applied, and the transmission is in NEUTRAL.
Feature inputs:
^ Throttle position information (transmitted from PCM)
^ MSS
^ Gear motor encoder contact plate position inputs A, B, C, D
^ Wheel speed signal (transmitted from ABS module)
^ Brake ON/OFF (BOO) switch input (battery voltage when brake pedal is depressed, open circuit when not activated)
Feature outputs:
^ 4WD shift motor outputs
^ 4WD LOW switch indicator
^ Solid state clutch (pulse width modulated signal)
Inspection and Verification Electronic On-Demand
1. Verify the customer concern.
2. Visually inspect for obvious signs of mechanical or electrical damage.
Visual Inspection Chart
3. If an obvious cause for an observed or reported concern is found, correct the cause (if Possible) before proceeding to the next step.
4. If the cause is not visually evident, connect the diagnostic tool to the data link connector and select the vehicle to be tested from the diagnostic tool menu. If the diagnostic tool does not communicate with the vehicle:
^ check that the program card is correctly installed (if using NGS+).
^ check the connections to the vehicle.
^ check the ignition switch position.
5. If the diagnostic tool still does not communicate with the vehicle, refer to the diagnostic tool operating information.
6. Carry out the data link diagnostics test. If the diagnostic tool responds with:
^ SCP+, SCP-, UBP or CAN or ISO circuits fault; all electronic control units no response/not equipped, Refer to Information Bus.
^ No response/not equipped for Powertrain Control Module (PCM). Refer to Information Bus to diagnose module communication problems.
^ No response/not equipped for 4x4 module (PCM), GO to Pinpoint Test A. Test A: No Communication With the 4X4 Module (PCM)
^ System passed, retrieve and record the continuous Diagnostic Trouble Codes (DTCs), erase the continuous DTCs and carry out the self test diagnostics for the 4x4 module (PCM).
7. If the DTCs retrieved are related to the concern, go to the 4x4 Module (PCM) DTC Index to continue diagnostics. Diagnostic Trouble Code Tests and Associated Procedures
8. If no DTCs related to the concern are retrieved, GO to Symptom Chart. Symptom Related Diagnostic Procedures